Mucosal surfaces are continuously exposed to harmless foreign antigens such as those from food and commensal bacteria, referred to as environmental antigens. The immune system should be tolerant to these environmental antigens, much as it is to self-antigens. While some mechanisms that regulate tolerance to self and environmental antigens may be shared, others may be unique. We have identified an E3 ubiquitin ligase adaptor, known as Nedd4-family interacting protein 1 (Ndfip1), that regulates T cell tolerance to environmental antigens in mice and man. We recently showed that mice lacking Ndfip1 develop atopic inflammation in lung, skin and GI tract. Additionally, using single nucleotide polymorphism (SNP) analysis, we have identified polymorphisms within the locus that encodes Ndfip1 (located on human Chr5) that are more common in patients with asthma, atopic dermatitis (AD) and inflammatory bowel disease (IBD)2, thus supporting that Ndfip1 regulates atopic disease in both mice and man. Studying Ndfip1-/- mice, we have determined that T cells lacking Ndfip1 become activated and produce Th2 cytokines in response to environmental antigens. We hypothesize that Ndfip1 regulates T cell tolerance by promoting the differentiation of nave T cells into iTregs and by inducing antigen-unresponsiveness. In this proposal, we will 1) determine the mechanism by which Ndfip1 promotes iTreg differentiation by determining whether Ndfip1 is required for iTreg conversion in vitro and in vivo and resolving whether the defect in iTreg conversion of Ndfip1-/- T cells is cell intrinsic or due to their production of IL-4. We will also establish whether Ndfip1 is required for Itch ubiquitylation of TIEG1. We will also 2) test whether Ndfip1-/- T cells require CD28-costimulation to become activated in vivo, use altered peptide ligands in vivo and in vitro to establish whether Ndfip1-/- T cells are activated by lower affinity antigens than WT cells, and determine whether Ndfip1 promotes antigen unresponsiveness in wild type nave T cells by dampening TCR or IL-2R signaling. Finally, we will establish the mechanism(s) underlying these defects. We believe that Ndfip1-dependent pathways could be targeted therapeutically to treat atopic inflammatory diseases such as asthma as well as T cell mediated autoimmune disease. A long-term goal of the laboratory is to design pharmacological mimics of Ndfip1 that would promote its functions therapeutically. The studies we propose will help us toward this goal.